The desirability in polytetrafluoroethylene dispersion resins of uniform particle shape, large average particle size, narrow particle size distribution as well as high molecular weight has long been recognized by those skilled in the art. Prior attemps to make polytetrafluoroethylene dispersion resins having this combination of properties were unsuccessful. Known high molecular weight polytetrafluoroethylene dispersion resins having relatively large average particle size lack either one or both of uniform particle shape and/or narrow particle size distribution. On the other hand, known polytetrafluoroethylene resins having large average particle size in combination with uniform particle shape and narrow particle size distribution have relatively low molecular weight, hence reduced thermal stability, which severely limits their use in many applications.
In the past, many methods have been proposed for making polytetrafluoroethylene dispersion resins of large average particle size, such as polymerizing to higher solid content; initiating the polymerization at temperatures below those employed during subsequent portions of the reaction period; conducting the polymerization under reduced agitation; reducing the concentration of dispersing agent; conducting the polymerization in the presence of small proportions of dissolved polyvalent metallic cations; or by "seeding" the polymerization medium by inclusion therein of "seeds" of dispersed polytetrafluoroethylene particles. All of these methods, unfortunately, suffer from one or more of the following disadvantages: lowered average molecular weight of the product; lowered overall space time yield; increased amount of coagulation; and wide variations from batch to batch in particle size and particle size distribution. None of the known methods for making polytetrafluoroethylene dispersion resins reproducibly allows production of resins having, in combination, uniform particle shape, large average particle diameter, narrow particle size distribution and high molecular weight, and none of these known methods allows production of polytetrafluoroethylene dispersion resin having the advantageous combination of properties possessed by the resins of the present invention.
Production of dispersion polytetrafluoroethylene resins of large, uniform particle size has heretofore been considered desirable because they were thought to have improved extrusion characteristics. While the novel resins of the present invention, which combine large average particle size and uniformity of particle shape with narrow particle size distribution, have good extrusion characteristics when "paste extruded" after admixture with lubricants, such as deodorized kerosene, in amount of about 20% by weight in the manner known to those skilled in the art, their extrusion characteristics do not appear to be substantially superior to those of known resins. However, the resins of the present invention have dispersion characteristics which are substantially superior to those of known products. Aqueous dispersions of the product of the present invention, as obtained from the polymerization process, can be concentrated by simple procedures to form stable concentrated aqueous dispersions containing more than about 75% by weight of polytetrafluoroethylene resin. Stable concentrated aqueous polytetrafluoroethylene dispersions of such high concentrations have heretofore not been available.